Wiped out how?

Working in Florida in late January (see my post about that issue) and encountered WiFi problems at the work site and also at the hotel. I foolishly, as it turned out, contacted my Internet provider (Spectrum/TWC) and asked for help. While they did not solve the problem, the support people managed to wipe out my inbox and my sent folders. Since my most recent backup was in late October and some of the more recent material could be retrieved from the ISP server, I figured I could work around this problem.

  I have the Thunderbird files backed up on an exterior hard drive and hoped to just do a copy and past into an "older inbox" folder that I could assess if I needed to find something.

Alas, I can't figure out how to copy and paste it. Is that possible?

BTW: While Spectrum/TWC did not solve the WiFi problem, I got around it by finding another network at the work site (Gulfstream Park). So...if I had not panicked and reached out to the ISP support people, I would not have lost that material.